Distributed Control Systems.
The concept of distributed industrial control systems, which have both a hardware and software component, is known in the art. Distributed control has been used since companies began installing programmable logic controllers (“PLCs”) to manage independent parts of a factory floor. PLCs are used in industrial control systems to provide coordinated control of equipment, devices, and processes. PLCs generally comprise a central processing unit (“CPU”) and a plurality of input/output (“I/O”) modules having I/O connection terminals. PLCs are ordinarily connected to various sensors, switches, or measurement devices that provide inputs to the PLC and to relays or other forms of output to control the field equipment or other controlled elements.
As control technology evolved, the idea of islands of programmable controllers was discarded in favor of larger, centralized controllers. Industry is now moving back to a decentralized approach in which small, intelligent controllers gather data locally and share it across a network.
The move back to a decentralized approach may be explained by describing the disadvantages of using a centralized control system. First, a centralized control system has lower flexibility and scalability. The maximum number of I/O modules and therefore I/O connections that can be controlled is determined during design by the model of controller used. Second, because a single processor (CPU) and a given amount of memory are used for the entire system, any future additional device, or any change in the system's configuration, must consider these limitations. Alternatively, an oversized central controller must be chosen in advance. Third, the wiring requirements for a centralized system, wherein every device must be wired to a central controller, are extensive. Although the use of remote I/O modules can reduce wiring requirements, adding more I/O modules to an existing controller does not add processing power and memory, which are usually fixed.
With reference to FIG. 1, a conventional modern home automation system 20 allows the homeowner to electronically control lighting, air-conditioning and other energy consumption devices, security and alarm systems, home theater systems, irrigation and water systems, hot water supply systems, and other controllable systems. Conventionally, a dedicated device, e.g., a controller 21–27, is used to control each sub-application independently (a distributed approach). However, for a homeowner to make the most out of the overall system, he or she must combine certain conditions and link all the different sub-applications together.
For example, when the rain detector connected to the irrigation controller 22 detects rain, the system 20 should (1) instruct the irrigation controller 22 to skip the irrigation plan for the day and (2) instruct the air condition and climate controller 26 to turn on heating in the baby's room. Similarly, when the alarm system connected to the security controller 25 detects an intrusion, the system 10 should (1) instruct the security controller 25 to sound an alarm signal and dial “911,” (2) instruct the lighting controller 27 to turn on lights in the house in a sequence simulating a person walking, and (3) instruct the irrigation controller 22 to turn on the irrigation sprinklers around the house. Or, when no motion is detected for more than 15 minutes in a room by the motion detectors connected to the security controller 25, the system 20 should (1) instruct the home theater controller 21 to turn off working audio systems in that room, (2) instruct the lighting controller 27 to turn off lights in that room after 30 minutes unless instructed otherwise, and (3) instruct the air condition and climate controller 26 to turn off air condition in that room after 60 minutes unless instructed otherwise.
In order to perform these tasks in a typical distributed system, each controller 21–27 must be programmed independently. The linking conditions must be programmed in each controller that triggers a condition and in each controller that should react to the triggered condition. Further, a wiring or networking infrastructure 28 linking the controllers 21–27 must be provided.
Alternatively, a centralized system could be used wherein a single controller controls all the devices in the system. This would allow every condition to be set in one control program, but the wiring requirements in such a system would be much more extensive, and, importantly, the system would suffer from the lack of flexibility and scalability. Were an existing device to need functional expansion, or were any additional device to need control in the future, it would require additional I/O modules to be added to the controller (if technically possible) and would require additional control programming as well, which would increase the workload for the fixed central processor and memory.
For these and other reasons, today's automation world is moving towards distribution of control rather than building centralized systems. In addition, the industry is making use of small (including “micro” and “nano”) PLCs and exploiting the latest improvements in communications and network technologies which have allowed companies to migrate from a centralized structure to a true distributed architecture. Such improvements include embedded networking capabilities (especially Ethernet, TCP/IP based solutions), new powerful CPUs that make controllers faster and stronger, enhanced security features for decentralized architectures, the physical downsizing of controllers, and reduced pricing.
Further, today's world automation market is seeing several major trends. First, the demand for programmable logic controllers (PLCs) is growing as end users need more automation and are not able to afford machine downtime. Second, users are taking advantage of commercial networks to connect PLCs in their systems. Third, the future lies in distributed networking of powerful, low cost PLCs.
As described above, some of the factors driving decentralized control include reduced wiring, better reliability, aggregated computing power, scalability, and modularity (right sized solutions). Nonetheless, while the drive towards distributed networking and decentralized control has important advantages, it also has significant drawbacks. Particularly, each controller in a decentralized system typically must be programmed independently (creating islands of control code). Moreover, system designers and programmers must handle the communications, signal sharing, interlocking, and other data transfer requirements between the controllers. Maintenance is more complicated because global changes must be repeated in each component and because a single change requires modifications in more than one controller. Further, although a decentralized control system is scalable, modular, and benefits from aggregated computing power, the workload between the processors typically is not balanced and depends on the way the designer chooses to distribute the code among the various processors.
Thus, the trend toward decentralized control opens new opportunities and creates important challenges. Therefore, a need exists for a system and method that uses the advantages of a distributed control environment, without loosing the power of a centralized system in designing, right-sizing, programming, running, and synchronizing an application.
Wireless Communication.
Another cross industry trend is the use of wireless communication. Wireless technology is used (1) by retailers for mobile shopping, personal shopper applications, merchandising, delivering retail content to wireless devices, and in-store employee applications; (2) by the healthcare industry for physician practice management, pharmaceutical sales force automation, e-clinical trials, and hospital information systems; (3) by the financial services industry for on-line financial services, wealth management, customer loyalty tracking, and insurance claims; (4) by the utilities industry for providing field workers access to information (schematics, alerts), providing customers with account access, providing access to supply chain information from anywhere, and providing machine-to-machine communication; and (5) by the travel and transportation industry for flight confirmations, rapid check-in, airline/airport operations, route track and trace, and rail car management.
Looking carefully into wireless machine-to-machine communication applications reveals notable growth in recent years. For instance, there has been an emergence of (1) after sale services, such as when a commercial supplier of industrial heating and cooling systems remotely monitors usage rates and tracks warranty data; (2) extended operations capabilities, such as where the wireless transmission of production data eliminates the requirement for timely manual data collection across remote areas and reduces the opportunity for error in data entry; (3) cashless payment methods, such as where radio frequency identification (“RFID”) transponders facilitate toll collection and monitor highway usage patterns; and (4) secure identification capabilities, such as using “Smart Cards” and embedded “Smart Chips” for access control and the storage of personal records.
With respect to today's world automation market, there are several major trends relating to wireless communication. For example, technological improvements have led to using PLCs as transmitters for remotely communicating with field instrumentation. In addition, the use of peer-to-peer communications and wireless technology has increased as control is distributed to field devices. Specifically, the introduction of Ethernet radio modems is a growing trend in the PLC industry for providing flexibility to system design and reliable wireless communication between PLCs and between a PLC and a PC.
The requirements of the PLC world for reliable communication links results mainly from the need to share data, transfer signals, communicate with sensors, and communicate with host computers (either for remote control or for programming purposes). In a distributed environment, the communication link is a key component for the entire system.
Existing solutions are based mainly on wired communication standards accepted by the industry, ranging from simple 2-wire or 4-wire physical infrastructures such as RS-232 or RS-485, using protocols such as ModBus or others, to Ethernet physical infrastructures using TCP/IP, or other protocols based on TCP/IP. These wired solutions are used both in the field bus level (connecting PLCs to field instrumentation, inputs, and outputs) and in higher levels such as connecting PLCs to each other or to host systems.
The need to reduce or even eliminate expensive wiring requirements, while increasing flexibility and mobility, resulted in several existing wireless solutions, all using external or independent Radio Frequency (“RF”) modems as wireless communication adapters. These include, (1) license-free radio modems using ISM (industrial, medical, and scientific) frequency bands such as 433 Mhz, 900 Mhz, 2.4 Ghz, or 5.7 Ghz, which can be used in certain countries without a special license; (2) licensed radio modems, which are usually more powerful devices, are required for longer ranges, and use frequency bands that usually need operating licenses from local authorities; (3) wireless LAN modems, which are a more recent trend, use standards such as IEE802.11 (b, g) developed to provide wireless connectivity especially for mobile computing (notebooks, PDAs, etc.), and usually require one or more central access points to be present; and (4) cellular modems that use the cellular telephony infrastructure (usually nationwide) for data purposes. Cellular modems exist for all type of networks (GSM, GPRS, CDMA, TDMA, iDEN, etc.).
The primary drawbacks of these existing solutions are (1) the need to develop support for the wireless modem (rarely a standard feature in PLCs); (2) the need for radio operation licensing in certain cases (especially for long range operations); (3) the relatively high cost of the devices (required for every PLC that needs to communicated wirelessly); and (4) the high operating costs that are required when using a public wireless network (such as a cellular network), whether such costs are fixed or based on traffic.
With reference again to FIG. 1, in the home automation system 10 described above, a wiring infrastructure 28 is required to connect the controllers 21–27 to the “world” being controlled (inputs such as sensors and outputs such as relays controlling valves, lights, air-condition motors, etc.), as well as to connect the controllers 21–27 to each other in order to allow data and signal interchange or remote programming. Therefore, extensive, and expensive, network wiring is required. Further, any modification or expansion to the system 10 (e.g., when adding a new controller for a new application) requires an addition of network infrastructure 28.
Accordingly, a need exists for a system that eliminates the burden of such cabling requirements.
Internet-Based Communication Platforms.
Instant messaging services, such as those provided by AOL, ICQ, Microsoft, and Yahoo, provide a communication platform using a client program installed on each user's computer, based on prior registration to the service. The client program automatically notifies the service provider's server every time a user (the client program) is connected to the Internet. The server maintains a list of users currently online, their current Internet (IP) addresses, and their user profiles, and serves as a coordinator between users whenever a user initiates a communication with another listed user.
The actual communication is conducted directly between the users after getting all of the required information from the coordinating server. The communication can be a “chat” program, a “messaging” (email-like) program, a file transfer program, a video conferencing application, a shared game, or other application. Such “instant messaging” functionality would be useful in automation systems.
Therefore, a need exists for an automation and control system that provides “instant messaging” functionality between control elements.